1. Field of the Invention
This invention relates to a bonding tool for tape automated bonding, used in a process of producing semiconductor chips, and more particularly, it is concerned with a bonding tool using polycrystalline diamond as a coating of the tool end.
2. Description of the Prior Art
Lately, technical progress in the field of semiconductors has become remarkable and production of the appliances using IC or LSI has shown a yearly increase. In order to draw out the electrical properties these semiconductor elements have, it is required to bond these with metallic fine wires called metal-plated leads or bonding wires. As the metal to be bonded, there is ordinarily used Au or an Au-Sn alloy which is chemically stable and has high electric conductivity and a bonding method comprising thermocompression bonding by means of a bonding tool heated has widely been employed.
The bonding tool used in the above described thermocompression bonding system can broadly be divided into two categories as shown in FIG. 2 and FIG. 1.
FIG. 2 is a schematic view of a pulse heating system, in which nichrome, stainless steel, inconel or molybdenum is used by subjecting to instantaneous heat generation through passage of electric current. In this system, however, such a material meets with a problem that there occur often oxidation, baking and deformation of the lead thereof at a high temperature and accordingly, it is required to clean periodically the end thereof.
FIG. 1 is a schematic view of a constant heating system, in which a polished single crystal diamond or ruby is buried in the end of a shank having a cartridge heater incorporated, and which is characterized by a longer life than that of the tool of the pulse heating system, in particular, in the case of single crystal diamond. The preferential use of diamond is due to the fact that diamond does not meet with marked thermal deterioration in the air at a temperature of up to about 900.degree. C. and has low compatibility and little reactivity with Au-Sn. In a polished diamond single crystal, its surface state is so good as represented by a Rmax of at most 0.1 .mu.m and is hardly changed because of its high hardness. Because of this character, the alloy of Au-Sn melted during compression bonding hardly adheres to and remains on the surface of the diamond.
When using diamond having the highest thermal conductivity of all the existing materials as a material for the tool of the steady heating system, furthermore, the tool end can be heated at a desired temperature, e.g. 500.degree. to 600.degree. C. without excessively heating the heater, i.e. shank. However, diamond single crystal is expensive and actually, even a relatively cheap synthetic one can hardly be obtained with a large size such as several mm or more. It is assumed that in the near future, the step of thermocompression bonding a number of terminals will be increased and in this case, a material with a size of at least 10 mm is required.
A diamond sintered compact having a high heat resistance is disclosed in, for example, Japanese Patent Laid-Open Publication No. 114589/1978, in which an iron group metal binder is extracted by treatment with an acid to form pores. In this case, the surface state cannot be rendered good even by polishing because of the presence of the pores so that an Au-Sn alloy tends to adhere thereto during use.
A pore-free, heat-resisting diamond sintered compact is disclosed in Japanese Patent Laid-Open Publication Nos. 161268/1984 and 33865/1986, in which the binder materials are composed of Si, SiC and Ni-Si alloys. In this case, the surface state after polishing is also unsatisfactory, since the hardness of the binder materials is lower than that of diamond.
A sintered compact containing no binder material and consisting of only polycrystalline diamond is considered most desirable as to heat resistance, hardness, thermal conductivity, surface roughness, etc. Thus, it has been proposed to sinter only diamond powder at an ultra-high pressure, but a composite material of diamond-graphite can only be obtained by this method, since diamond grains themselves are hard to deform so that pressure cannot be transmitted to gaps among the grains and consequently, graphitization takes place.
On the other hand, a technique of producing a binder-free polycrystalline diamond compact by a gaseous phase synthesis method has lately made rapid progress and it is considered effective to apply this technique to the bonding tool. As well known in the art, a cutting tool has been put to practical use, which is produced by depositing a thin film of diamond on a substrate of cemented carbide or tungsten by a gaseous phase synthesis method, but even if this is applied to production of the bonding tool, good results cannot be obtained because the bonding strength of the film is too low to prevent this from stripping and occurrence of cracks.
Furthermore, the bonding tool of this kind has other problems. In the bonding tool, a shank and tool end are bonded, for example, by a silver-brazing method, gold-brazing method, thermocompression or sintering bonding method using gold. On the other hand, there are a number of varieties in IC and LSI products, which differ in the number of leads connected and the shape of semiconductors every product. Thus, it is required to prepare an exclusive bonding tool for each IC or LSI product and accordingly, problems arise that the production cost is increased, more time is taken for exchanging the bonding tool and management of the bonding tool becomes complicated.